1. Field of the Invention
This invention relates to a method and apparatus for the analysis of the composition of materials, and to the analysis of the composition of semiconductor materials using diffraction.
By “composition” it is meant the chemical composition.
2. Discussion of Prior Art
Many methods exist by which the chemical composition of a material may be analysed, e.g. characteristic x-ray emission or photoluminescence. Various diffraction methods of, for example, x-rays, electrons or neutrons also exist. In these the material lattice parameter (i.e. the spacing between lattice planes) is commonly measured. This can be related to the material composition by, for example using Vegard's law, and therefore the composition can be estimated from such measurements. However, a number of problems exist with this method. For some materials, e.g. SiGe, Vegards law is thought not to hold, and reliance on this to determine the composition can give rise to erroneous results.
It is desirable to be able to analyse the composition of semiconductor materials, e.g. III-V semiconductor materials, particularly III-V quaternary semiconductor materials. The wide variety of band gaps found in these compounds makes them suitable for the production of light emitting diodes and laser devices over a wide range of wavelengths. The ability to choose compounds with the desired band gaps and with minimal lattice mismatch to allow the growth of strain free structures offers considerable freedom to device designers. However, growth of quaternary III-V compounds with precisely controlled chemical compositions is very challenging, with a major limitation being the lack of a reliable and simple method for determining the chemical composition of these materials. The standard technique used to determine the composition of ternary III-V epilayers is measurement of x-ray diffraction rocking curves. This method utilises the x-ray rocking curve from a layer to determine its lattice constant which may then be related to the elemental ratios of the chemical elements present via a Vegard's law type relationship. For the case of quaternary compounds this technique does not provide a unique solution, since neither the group III or group V elemental ratios are known. Thus other techniques such as photoluminescence have traditionally been employed to provide additional information allowing quaternary compositions to be estimated. However, for such techniques the exact relationship between composition and band gap is often not known and effects such as compositional ordering also complicate interpretation. More direct techniques for compositional analysis, such as energy dispersive x-ray (EDX) analysis and wavelength dispersive x-ray (WDX) analysis, are also limited due to the strong spectral interference between many of the x-ray lines and their inability to measure thin and buried layers. This latter point is of particular importance in the case of Al containing layers which oxidise rapidly on exposure to air and must therefore be capped.